To optimise the performance of an electron multiplier, it is often desirable to have a large sensitive input area so that particles can be detected which are incident over a large area. This requirement often results in a mis-match between the desired sensitive input area and the sensitive area of the amplifying section of the electron multiplier (which can be much smaller). In this case it is desirable to include a focussing element, usually referred to as a focussing lens, between the device's input aperture and its amplifying section.
As well as enabling the detector to have a large sensitive input area, a number of additional requirements for the focussing lens will be necessary if the device is to be used for special applications such as time-of-flight mass spectrometry (TOF-MS). For TOF-MS applications it is critical to accurately measure the arrival time of the ions that are detected over the sensitive input area. To achieve this objective, the focussing lens, at least in a preferred form, should be such as to contribute little or no distortion to the relative measured arrival times of input ions. Expressed another way, if multiple ions arrive at the detector and are spread uniformly over the input area and are all coincident in time, the electrons (resulting from these ions) exiting the focussing lens should all impact the first dynode of the amplifying section substantially in coincidence.
The amplifying section may be a discrete dynode electron multiplier, a continuous dynode electron multiplier, a micro channel plate, a micro sphere plate, a focussed mesh detector, a magnetically focussed electron multiplier, a magnetic/electrostatic electron multiplier (also known as a cross field detector) or any other device that can be used to amplify the signal electrons.
A focussing lens typically includes an ion impact plate as the first element of the lens assembly. This ion impact plate is an integral component of most ion detectors and has the function of converting the input ions, to be detected, into electrons. The emission of low-energy secondary electrons from the impact plate, typically as a beam of electrons, is the desired response to the plate being struck by sufficiently energetic particles, and forms the principal signal to be amplified by the detector. In addition to the desired secondary electrons, the incoming signal ions may cause numerous other interactions that may generate particles within the detector. These particles include:                a) Grid ions: Ions that are emitted from the detector's entry grid as a result of an impact on the grid by a signal ion. They can be positive or negative, low energy or high energy.        b) Grid electrons: Electrons that are emitted from the detector's entry grid as a result of an impact on the grid by a signal ion.        c) Grid neutrals: Neutral atoms or molecules that are emitted from the detector's entry grid as a result of an impact on the grid by a signal ion.        d) Impact plate ions: Secondary ions that are emitted from the detector's impact plate as a result of an impact on the plate by a signal ion. They can be positive or negative, low energy or high energy.        e) Impact plate neutrals: Neutral atoms or molecules that are emitted from the detector's impact plate as a result of an impact on the plate by a signal ion.        
For time-of-flight mass spectrometry all particles resulting from these interactions within the detector (apart from secondary electron emission from the impact plate) generate unwanted artefact signals in the detector output. Such artefacts are usually seen as unwanted small peaks in the mass spectrum, which are not coincident with the primary signal associated with the incoming ion, and thus add confusion when interpreting the spectrum. It is desirable to eliminate or minimise these artefacts so that they no longer unduly interfere with the intended signal.
In short, a primary objective of the invention is therefore to spatially focus electrons resulting from the impact of the particles to be detected, sometimes referred to as the signal or signal carrying particles or signal or signal carrying electrons, without degradation of the timing information.